The objectives of this study are to characterize the gastrointestinal abnormalities that occur in CVID patients and correlate this with the immunophenotype and cytokine secretion of peripheral blood and lamina propria lymphocytes and monocytes. CVID patients with gastrointestinal symptoms of malabsorption/maldigestion and chronic diarrhea will be targeted for study. We will also include a group of patients without gastrointestinal symptoms to provide an estimate of background prevalence and severity of gut abnormalities. Subjects will undergo a standard immunologic workup including peripheral blood lymphocyte marker phenotyping and cytokine responses as well as tests of gastrointestinal absorption, examination of gut histology by endoscopic biopsy, and measurement of gut mucosal cytokine expression. Analysis variables will include clinical (weight, stool frequency, results of gut absorption tests), laboratory (lymphocyte and cytokine assays), and gut abnormalities (histology scores and specific lesions).